Method for blow molding hollow articles

ABSTRACT

In a blow molding process, a fused thermoplastic polymer parison is interposed between a pair of split molds. The split molds are closed around the parison to seal the ends. A small diameter hollow needle is inserted through one of the split molds and fluid is blown into the parison forming a balloon. After the balloon begins to form, a large diameter hollow needle is inserted while internal pressure is applied to the parison via the small diameter hollow needle, and a large amount of fluid is blown in. This process is highly productive and reduces the conventional blow molding cycle by 60 percent due to the rapid cooling of the hollow article.

BACKGROUND OF THE INVENTION

The present invention relates to an improved method and device for blowmolding and shaping hollow articles. In particular, the presentinvention relates to a highly productive method and device for directblow molding which allows rapid cooling of articles shaped by parisonexpansion or blowing.

Blow molding is a technique adopted from the glass industry for moldingplastic bottles and other articles from thermoplastic material. Thedirect blow molding process has been widely used for the shaping ofhollow articles made of thermoplastic synthetic resins. It consists ofblowing a thin balloon of molten thermoplastic material against theinside walls of a mold and chilling it to a rigid solid. In its mostcommon form, this process includes extruding or injecting a parisondownward between the opened halves of a mold, closing the mold to pinchoff and seal the parison at top and bottom, injecting air or other fluidthrough a needle inserted through the parison wall, cooling the mass incontact with the mold, opening the mold, and removing the formedarticle.

It is difficult to insert a thick needle into the parison to inject airor other fluid because the parison is relatively soft at hightemperatures. It is therefore necessary to use a hollow needle having athin and sharp tip. Air is a fluid and as such is limited in its abilityto flow through an orifice. If the air entrance channel is too small,the required blow time is excessively long or the pressure exerted onthe parison is inadequate to reproduce the surface details of the mold.Furthermore, while a small amount of air may be adequate for expandingthe parison, it might not be enough for cooling the expanded hollowarticle. Although small articles can be shaped this way, it is notpossible to shape large articles.

In the direct blow molding method, a technique used to prevent cosmeticdamage to the shaped article includes forming an exhaust opening throughan unnecessary section of the article that is cut off after formation toserve as an opening (referred to as the flash section). Anothertechnique includes inserting the hollow needle from an angleperpendicular to the main axis of the parison. In blowing air into thehollow article from this position, it is desirable to blow in thedirection of the body of the article, i.e., downward along the axis ofthe parison, so that the air can flow easily. However, thin needles canonly be designed with an opening at the tip of the needle due to thesize constraints. A needle bent to a 90 degree angle cannot penetratethe parison. Thus, it is only possible to blow air in a directionperpendicular to the direction of the parison axis, and effectivecirculation of the air within the parison is prevented. Since a largeamount of air for cooling can not be blown into the parison, more timeis required for cooling the article.

Cooling is particularly important because it consumes much of the cycletime and therefore bears on product economics. Cooling can take as muchas two thirds of the entire "mold-closed" time in a cycle. Results arebest when uniform temperatures are maintained throughout the mold.Standard cooling techniques are directed towards either external systemsor internal systems or a combination thereof. External systems cool themold by circulating coolants around the outside or through the walls ofthe mold. Mold cooling can be improved by increasing the rate of coolantflow through the mold or by making the mold of material with better heattransfer.

Internal systems rely on injecting fluids such as air, a mixture of airand water, or carbon dioxide into the blown part to cool the inside ofthe parts while they are in the mold. Typical commercial methods include(1) injecting liquid carbon dioxide into the blown part, followed byvaporization, superheating, and exhaustion of the coolant as a gasthrough the blow pin, (2) injecting highly pressurized moist air intothe blown part where it expands to normal blow pressures and produces acooling effect, (3) passing air through a refrigeration system and intothe hot parison, and (4) cycling normal plant air into and out of theblown parts by a series of timers and valves. The size of the hollowneedle used for injecting the fluid limits both the speed of the processand the size of articles made using the process.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amethod and device for the direct blow molding of hollow articles using alarge diameter hollow needle in addition to a conventional smalldiameter hollow needle. A further object of the present invention is toprovide a highly productive method and device for shaping hollowarticles made of thermoplastic polymers.

When shaping a hollow article by the direct-blow method according to thepresent invention, blowing fluid through a large diameter hollow needleallows a large amount of fluid to be blown into the parison and provideeffective cooling. This makes it possible to form large articles. Inaddition, using a large diameter hollow needle allows the position ofits orifice to be changed freely, thus allowing efficient expansion ofthe parison. The orifice need not be located at a tip of the largediameter hollow needle.

Large diameter hollow needles can not directly puncture a parison wallbecause the parison wall is relatively soft at high temperatures. Whenthe large diameter needle is forced against the outside of the parisonwall, the softness of the thermoplastic material permits substantialdeformation. However, by initially using a small diameter hollow needleto puncture the parison and blow fluid into the parison, the parison isgiven internal pressure. This internal pressure counteracts theelasticity of the balloon wall and thereby makes it possible to insertthe large diameter hollow needle shortly after the small diameter hollowneedle is inserted without producing undesired distortion. By using acombination of a large diameter hollow needle and a small diameterhollow needle, the present invention enables the use of larger diameterneedles than could be used in the prior art.

Using a large diameter needle allows a large amount of fluid to berapidly blown into the parison. This permits the rapid expansion of theparison as well as adequate cooling, thus making it possible to improvethe formation process. Since a large amount of fluid can be blown in,large bottles can be formed quickly and efficiently.

A parison having a bottom or a tube-shaped parison can be used as theparison in the present invention. The flash section is arranged in thearea above the section that forms the opening of the article. Arranginga protrusion or cavity in the flash section where the large diameterhollow needle is inserted keeps this section thin during blowing, whichis especially desirable because it makes inserting the large diameterhollow needle easier.

Briefly stated, in a blow molding process, a fused polymer parison isinterposed between a pair of split molds. The split molds are closedaround the parison to seal the ends. A small diameter hollow needle isinserted through one of the split molds and fluid is blown into theparison forming a balloon. After the balloon begins to form, a largediameter hollow needle is inserted while internal pressure is applied tothe parison via the small diameter hollow needle, and a large amount offluid is blown in. This process is highly productive and reduces theconventional blow molding cycle by 60 percent due to the rapid coolingof the hollow article.

The present invention also provides a device for blow molding hollowarticles including a pair of split molds arranged so they can be freelyopened and closed and having an open position for receiving a parisonand a closed position for forming a mold. The mold has an upperpinch-off section, a flash section, a cavity, and a lower pinch-offsection. The device includes a plurality of drive devices for insertingand retracting a plurality of hollow needles into and out of the parisonand for supplying a fluid to each of the hollow needles. The drivedevices are arranged in a first one of the pair of split molds in anarea corresponding to the flash section of the mold. The device alsoincludes means for exhausting the fluid from the parison.

According to a feature of an embodiment of the present invention, eachof the drive devices includes a fluid cylinder containing one of thehollow needles, first and second fluid supply openings disposed on aside of the fluid cylinder and having an open position and a closedposition whereby the first fluid supply opening being closed and thesecond fluid supply opening being open causes the hollow needle in thefluid cylinder to be inserted into the parison and the fluid to flowthrough the hollow needle into the parison, and whereby the first fluidsupply opening being open and the second fluid supply opening beingclosed causes the hollow needle to be retracted from the parison.

According to an embodiment of the present invention, a device for blowmolding hollow articles includes a pair of split molds arranged so theycan be freely opened and closed and having an open position forreceiving a parison and a closed position for forming a mold which hasan upper pinch-off section, a flash section, a cavity, and a lowerpinch-off section. A plurality of hollow needles are arranged in one ofthe pair of split molds in an area corresponding to the flash section ofthe mold. The device includes means for controlling each hollow needlewhereby each hollow needle is inserted into the parison and retractedfrom the parison, means for supplying a fluid to each of the hollowneedles, and means for exhausting the fluid from the parison.

According to a feature of the invention, the plurality of hollow needlesincludes a small diameter hollow needle and a large diameter hollowneedle.

According to a feature of the invention, the means for controllingincludes a first cam linked to the small diameter hollow needle wherebythe small diameter hollow needle is inserted into the parison andretracted from the parison, a second cam linked to the large diameterhollow needle whereby the large diameter hollow needle is inserted intothe parison and retracted from the parison, and the first and secondcams are linked.

According to an embodiment of the invention, a device for blow moldinghollow articles includes a pair of split molds arranged so they can befreely opened and closed and having an open position for receiving aparison and a closed position for forming a mold, the mold having anupper pinch-off section, a flash section, a cavity, and a lowerpinch-off section, a small diameter hollow needle and a large diameterhollow needle arranged in one of the pair of split molds in an areacorresponding to the flash section of the mold, means for controllingthe small diameter hollow needle whereby the small diameter hollowneedle is inserted into the parison and retracted from the parison,means for controlling the large diameter hollow needle whereby the largediameter hollow needle is inserted into the parison and retracted fromthe parison, means for supplying a first fluid to the small diameterhollow needle, means for supplying a second fluid to the large diameterhollow needle, and means for exhausting the first and second fluids fromthe parison.

The present invention provides for a method for blow molding hollowarticles which includes interposing a thermoplastic parison between apair of split molds, closing the pair of split molds thereby sealingboth ends of the parison, inserting a small diameter hollow needle intothe parison, blowing a fluid through the small diameter hollow needleinto the parison thereby forming a balloon by expanding the parison intoa hollow space inside the pair of split molds, inserting a largediameter hollow needle into the balloon, blowing the fluid through thelarge diameter hollow needle into the balloon, circulating the fluidinside the parison by forming an exhaust opening in the parison, coolingthe balloon thereby forming the hollow article, exhausting the fluidfrom the balloon, retracting the small diameter hollow needle from theballoon, retracting the large diameter hollow needle from the balloon,opening the pair of split molds, and removing the hollow article fromthe pair of split molds.

The above, and other objects, features and advantages of the presentinvention will become apparent from the following description read inconjunction with the accompanying drawings, in which like referencenumerals designate the same elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a section view of an embodiment of the present inventionshowing a split mold in an open state and hollow needles in an inactive(retracted) position.

FIG. 2 is a section view of an embodiment of the present inventionshowing the split mold in a closed state and the hollow needles in anactive (inserted) position.

FIG. 3 is a detailed section view showing both of the hollow needles inan inactive position.

FIG. 4 is a detailed section view showing the small diameter hollowneedle in the active position with a fluid being blown therethrough andthe large diameter hollow needle in the inactive position.

FIG. 5 is a detailed section view showing both hollow needles in theactive position with fluid being blown through both needles.

FIG. 6 is a detailed section view showing the small diameter hollowneedle beginning to retract while fluid is still being blown through thelarge diameter hollow needle.

FIG. 7a is a side section view showing a cam control mechanism forcontrolling the timing of the movement of the hollow needles.

FIG. 7b is a section view of the cam movement of part A of FIG. 7a.

FIG. 7c is a section view of the cam movement of part B of FIG. 7a.

FIG. 8 is a timing diagram showing the relation between the cammovements shown in FIGS. 7a-7c and the injecting of fluid through thehollow needles.

FIG. 9 is a valve diagram showing a control mechanism according to anembodiment of the present invention.

FIG. 10 is a detailed section view of the hollow needles inserted intothe parison adjacent each other according to an alternative preferredembodiment.

FIG. 11 is a section view depicting a thinning cavity inside the flashcavity according to an alternative embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a pair of split molds 1a and 1b, which can beopened and closed, include pinch-off sections 3a and 3b, cavity sections4a and 4b, and flash cavity sections 5a and 5b. Pinch-off sections 3aand 3b seal parison top 2a when the split molds close. Parison bottom 2bis already closed from the extrusion process that produces parison 2.Small diameter hollow needle 6 and large diameter hollow needle 7 arearranged at the top of split mold 1b such that, when inserted intoparison 2, they penetrate flash cavity section 5b. Fluid supplymechanism 10 and control device 11 allow for air or other fluid to beinjected into parison 2 through small diameter hollow needle 6 and largediameter hollow needle 7 at the proper times. Exhaust opening formationmechanism 8 allows any air trapped between parison 2 and a side ofclosed split molds to vent, and also allows the injected air or otherfluid to vent to the outside of parison 2 and mold after expansion ofparison 2 is completed. After the hollow article cools sufficiently,knockout pin 9 removes the cooled hollow article from split mold 1b assplit molds 1a and 1b separate.

Referring to FIG. 2, when split molds 1a and 1b are closed, cavitysections 4a and 4b form cavity 4 and flash cavity sections 5a and 5bform flash cavity 5. Pinch-off sections 3a and 3b seal parison top 2aand the portion of the extruded tube (not shown) that becomes parisonbottom 2b of the next parison to enter the mold. The air or other fluidblown through small diameter hollow needle 6 and large diameter hollowneedle 7 inflates parison 2 against the sides of the mold to form thehollow article. Exhaust opening formation mechanism 8 allows theinjected air or other fluid to vent to the outside of parison 2 and themold. After the hollow article cools sufficiently, knockout pin 9removes the cooled hollow article from split mold 1b as split molds 1aand 1b separate. Flash section 5c is then trimmed. Flash section 5c is asection formed on an upper opening of the hollow article where smalldiameter hollow needle 6, large diameter hollow needle 7, exhaustopening formation mechanism 8, and knockout pin 9 are formed. Sinceflash section 5c is cut off after the blowing process, the final articlebears no cosmetic damage caused by the production process.

Small diameter hollow needle 6 and large diameter hollow needle 7 moveforward and backward between the inactive or retracted position shown inFIG. 1 and the active position shown in FIG. 2. Pressurized fluid supplymechanism 10 supplies pressurized fluid to parison 2 through smalldiameter hollow needle 6 and large diameter hollow needle 7. Controldevice 11 controls the timing of the movement of the hollow needles andthe action of the fluid from a pressurized fluid source (not shown inthe drawings) via pressurized fluid supply mechanism 10. In thisembodiment, the fluid is used to move the hollow needles between theactive and inactive positions in addition to being used to inflateparison 2.

Referring to FIG. 3, small diameter hollow needle 6 and large diameterhollow needle 7 are forced to the rear of their respective fluidcylinders by fluid flowing from fluid supply openings 14a and 14b at thefront of the cylinders. Small diameter hollow needle 6 and largediameter hollow needle 7 are in the inactive position. Small diameterhollow needle 6 and large diameter hollow needle 7 are held within fluidcylinders 13a and 13b respectively, which are included in drive devices12a and 12b respectively. The hollow needles move forward and backwardwithin these cylinders. Drive devices 12a and 12b also include fluidsupply openings 14a, 14b, 15a, and 15b, which are formed on the sides offluid cylinders 13a and 13b for supplying pressurized fluid. Fluidsupply openings 14a and 14b pass through the front side of fluidcylinders 13a and 13b. Fluid supply openings 15a and 15b pass throughthe rear side of fluid cylinders 13a and 13b. The fluid supplied throughthese fluid supply openings either flows within the cylinders or flowsfrom the back of each hollow needle to the front and into parison 2.

Drive device 12a for small diameter hollow needle 6 and drive device 12bfor large diameter hollow needle 7 are arranged so that they can move inparallel. This permits the control mechanism and the pressurized fluidsupply mechanism to be used in common, and also contributes tosimplifying the devices required for supplying fluid and moving thehollow needles.

Referring now to FIG. 4, fluid from fluid supply opening 14a is stoppedand fluid from fluid supply opening 15a is started. Small diameterhollow needle 6 moves forward toward the parison and penetrates it inflash cavity 5. Once the rear end of small diameter hollow needle 6clears fluid supply opening 15a, fluid flows through small diameterhollow needle 6 into parison 2, thereby creating internal pressure inballoon 22 inside the mold. Large diameter hollow needle 7 remainsretracted since fluid is still supplied from fluid supply opening 14bwhich forces large diameter hollow needle 7 to the rear.

Referring now to FIG. 5, fluid from fluid supply opening 14b to largediameter hollow needle 7 is stopped and fluid from fluid supply opening15b is started. This causes large diameter hollow needle 7 to moveforward to penetrate parison 2. Since the parison is given internalpressure which counteracts the elasticity of the wall of balloon 22, itis possible to insert large diameter hollow needle 7 shortly after smalldiameter hollow needle 6 is inserted. Once the rear end of largediameter hollow needle 7 clears fluid supply opening 15b, fluid flowsthrough large diameter hollow needle into parison 2 and balloon 22,thereby greatly increasing the total rate of flow of fluid insideparison 2.

Referring now to FIG. 6, while fluid is being supplied to large diameterhollow needle 7 from fluid supply opening 15b, the fluid from fluidsupply opening 15a to small diameter hollow needle 6 is stopped andfluid from fluid supply opening 14a is started. This causes smalldiameter hollow needle 6 to move to the rear. At the completion of theblowing cycle, large diameter hollow needle 7 retracts in a similarmanner (not shown).

When expansion of the parison is almost complete, a small diameterexhaust opening is formed in order to circulate fluid blown into theparison. As shown in FIGS. 1-2, exhaust opening formation mechanism 8includes a nozzle which is inserted into flash section 5c, where smalldiameter hollow needle 6 and large diameter hollow needle 7 areinserted. Exhausting the fluid through exhaust opening formationmechanism 8 while fluid is still being fed through large diameter hollowneedle 7 increases the circulation of fluid inside the hollow articleand enhances the cooling process. However, as noted in Japaneselaid-open patent 59-3260, it is also possible to form an exhaust openingusing the pressurized fluid within the upper pinch-off section of thesplit mold. After the fluids are exhausted, the shaped hollow article isremoved from the mold using a knock-out pin such as knock-out pin 9.This completes the shaping of the hollow article.

Positioning large diameter hollow needle 7 more toward the parison bodyside than small diameter hollow needle 6, which is roughly centeredwithin the diameter of flash cavity 5, makes it possible for the largeamount of fluid blown in through large diameter hollow needle 7 tocirculate more efficiently.

The well known small diameter needles used in conventional direct blowmethods can also be used as the small diameter needle in the presentinvention. The orifice diameter of small diameter hollow needle 6 variesaccording to the size of the article to be formed, but generally thediameter is between 0.5 mm and 5 mm, and in particular, 1.5 mm to 3 mmis desirable.

The orifice diameter of large diameter hollow needle 7 is 3 to 15 timeslarger than that of small diameter hollow needle 6, and in particular, afactor of 4 to 10 is desirable. In other words, based on a conventionalsmall diameter hollow needle 6 size between 0.5 mm and 5 mm, the orificediameter of large diameter hollow needle 7 is 1.5 mm to 75 mm, and inparticular, a diameter of 2.0 mm to 50 mm is desirable. A smallerdiameter needle does not supply enough fluid for cooling and a largerdiameter needle is very difficult to insert into the parison.

Unlike small diameter hollow needle 6, large diameter hollow needle 7can be constructed so that the orifice through which fluid is blown islocated other than at the tip of the needle. Thus, the orifice can bearranged at the side of large diameter hollow needle 7 instead of at theend. FIGS. 5-6 depict orifice 21 located on the side of large diameterhollow needle 7 which permits fluid to be blown into parison 2 in thedirection of balloon 22, i.e., in the vertical axis direction. Thisarrangement allows the fluid to circulate efficiently within the parisonbody.

Referring to FIGS. 7a through 7c, an alternative embodiment is shownusing cams instead of fluid pressure to control small diameter hollowneedle 6 and large diameter hollow needle 7. Cam 16a controls themovement of small diameter hollow needle 6 via pilot valve 17a and cam16b controls the movement of large diameter hollow needle 7 via pilotvalve 17b. Pilot valves 17a and 17b are connected to a pressurized fluidsupply (not shown). Cam 16a and cam 16b rotate in the directionindicated by the arrows in FIGS. 7b and 7c respectively.

The split mold closing position is 0 degrees (not shown). Small diameterhollow needle 6 and large diameter hollow needle 7 do not move whilecams 16a and 16b are between 0 degrees and point A. When cam 16a reachespoint A, cam roller 28 opens pilot valve 17a and small diameter hollowneedle 6 is inserted into the parison by the action of the pressurizedfluid. Fluid is blown into the parison from small diameter hollow needle6 while cam 16a is between point A and point B thus applying an internalpressure to the parison. When cam 16b reaches point C, cam roller 29opens pilot valve 17b and large diameter hollow needle 7 is insertedinto the parison by the action of the pressurized fluid. Fluid is blowninto the parison from large diameter hollow needle 7 while cam 16b isbetween point C and point D.

Referring also to FIG. 8, the relation between the cam movements and theclosing and opening of split molds 1a and 1b is depicted. The split moldclosing position is 0° and the split mold opening position is 360°. Whencam 16a reaches point A, small diameter hollow needle 6 is inserted intoparison 2 and fluid is blown in. Fluid flows through small diameterhollow needle 6 during a time L. After a short time S, during whichfluid is blown in through small diameter hollow needle 6 only, cam 16bhas rotated to position C, and large diameter hollow needle 7 isinserted into parison 2. Large diameter hollow needle 7 penetrates theparison wall because the internal pressure created by small diameterhollow needle 6 counteracts the elasticity of the balloon wall. Fluidflows through large diameter hollow needle 7 during a time Q. During atime W, fluid flows into parison 2 through both hollow needles. Time Wends when cam 16a reaches point B and small hollow needle 6 retracts.Fluid continues to flow into parison 2 through large diameter hollowneedle 7 until cam 16b reaches point D, at which point large diameterhollow needle 7 retracts.

The parison 2 expands during time L, forming the desired shape and size.For small articles, the inside of the shaped hollow article is cooledduring time Q by the fluid blown through large diameter hollow needle 7.For large articles, some expansion of parison 2 continues during part oftime Q, after which cooling occurs.

In the present invention, the time L is 0.4 to 12 seconds, and inparticular, 1 to 4 seconds is desirable. The time S is 0.2 to 2 seconds,and in particular, 0.5 to 1 second is desirable. If time S is shorterthan this interval, the internal pressure within parison 2 is inadequateto allow the insertion of large diameter hollow needle 7. If time S islonger than this interval, the production cycle is unnecessarily sloweddown.

Time Q is 5 to 30 seconds, and in particular, 10 to 20 seconds isdesirable. Time W is 0.2 to 10 seconds, and in particular, 0.5 to 5seconds is desirable. If time W is shorter than this interval, the fluidsupplied by large diameter hollow needle 7 may back up into smalldiameter hollow needle 6. If time W is longer than this interval, theproduction cycle is unnecessarily slowed down.

Referring now to FIG. 9, conventional valves and techniques are used forcontrolling the operation of the hollow needles according to anembodiment of the present invention. A pressurized fluid, such ascompressed air or dry nitrogen, flows from fluid supply 24 to pilotvalves 17a and 17b. Cam roller 28 controls pilot valve 17a, and in alike fashion, cam roller 29 controls pilot valve 17b. When pilot valve17a is activated, fluid flows through master valve 19, through controlvalve 20, and into fluid supply opening 15a. Small diameter hollowneedle 6 is thus moved forward. As pilot valve 17b is activated, fluidflows through master valve 6 and into fluid supply opening 15b. Largediameter hollow needle is thus moved forward.

When master valve 23 is activated by fluid from master valve 26, thefluid flow to small diameter hollow needle 6 is cut off. The role ofmaster valve 23 is to ensure fluid is blown through small diameterhollow needle for a short period even after fluid begins blowing throughlarge diameter hollow needle 7. This timing prevents the fluid insidethe parison from backing up into small diameter hollow needle 6.

After cam 16a rotates to point B as shown in FIG. 7b, the fluid flow tosmall diameter hollow needle is stopped. Fluid continues to flow throughlarge diameter hollow needle 7 until cam 16b rotates to point D in FIG.7c. The exhaust opening formation mechanism 8 shown in FIG. 2 retractsshortly after fluid begins blowing through large diameter hollow needle7, and the pressure of the fluid creates an opening in flash cavity 5c.Fluid continues to be blown through large diameter hollow needle 7 andcirculates inside the hollow article before being exhausted. This fluidcirculation cools the thermoplastic material quickly. After fluid is cutoff by pilot valve 17b, large diameter hollow needle 7 and smalldiameter hollow needle 6 retract.

Any conventional control techniques, such as pneumatic, hydraulic,electronic, or digital, may be used to ensure the proper timing of themovement of the hollow needles and the blowing of fluid through theneedles. For example, the timing of blowing the fluid through the hollowneedles can be controlled using electronic timers.

Referring to FIG. 10, an alternative embodiment is shown in which thedrive devices for the hollow needles are not parallel. When the drivedevices are parallel, the minimum distance between the hollow needlesequals the radius of drive device 12a plus the radius of drive device12b. Instead, the hollow needles are disposed at angles to each otherperpendicular to the article axis. Large diameter hollow needle 7 andsmall diameter hollow needle 6 are positioned so that they are angledwith respect to each other instead of parallel to each other. Thispositioning allows large diameter hollow needle 7 and small diameterhollow needle 6 to be inserted into flash section 5c of parison 2 atclose to the same longitudinal position on the article axis. The size offlash section 5c can thus be minimized. In an embodiment with large orbulky drive devices 12a and 12b, this arrangement allows the hollowneedles to be positioned much closer to each other along the articleaxis than would otherwise be possible.

A cycle begins when parison 2 is moved into mold 1. Parison guide pins30 and 31, shown extended for illustrative purposes only, are fullyretracted into their respective cylinders at the beginning of the cycle.In a similar fashion, small diameter hollow needle 6 is fully retractedinto drive device 12a and large diameter hollow needle 7 is fullyretracted into drive device 12b at the beginning of the cycle. Coolingpassage 32 allows cooling water to circulate inside the body of mold 1to reduce the temperature. Drive devices 12a and 12b include fluidsupply openings 14a, 14b, 15a, and 15b, which supply pressurized fluid.Fluid supply openings 14a and 14b pass through the front side of drivedevices 12a and 12b. Fluid supply openings 15a and 15b pass through therear side of drive devices 12a and 12b. Fluid flows through fluid supplyopenings 14a and 14b and does not flow through fluid supply openings 15aand 15b, thus keeping both needles in the inactive (retracted) position.

After parison 2 enters mold 1, parison guide pins 30 and 31 extend tohold parison 2 in place. After mold 1 is fully closed, parison guidepins 30 and 31 keep parison 2 centered while small diameter hollowneedle 6 penetrates it. Other conventional means can be used as parisonguides.

In the next step of the cycle, the fluid flowing through fluid supplyopening 14a is stopped and fluid flow through fluid supply opening 15ais started. Small diameter hollow needle 6 moves forward toward theparison and penetrates it. Once the rear end of small diameter hollowneedle 6 clears fluid supply opening 15a, the fluid flows through theinside of small diameter hollow needle 6 and into parison 2, therebycreating internal pressure inside parison 2. Large diameter hollowneedle 7 remains in its inactive position while fluid is supplied fromfluid supply opening 14b, thus forcing large diameter hollow needle 7 tothe inactive position.

In the next step of the cycle, the fluid through fluid supply opening14b to large diameter hollow needle 7 is stopped and fluid through fluidsupply opening 15b is started. This causes large diameter hollow needle7 to move forward to penetrate parison 2. The internal pressure providedby small diameter hollow needle 6 resists deformation of parison 2 whenlarge diameter hollow needle 7 is inserted. Once the rear end of largediameter hollow needle 7 clears fluid supply opening 15b, fluid passesthrough large diameter hollow needle 7 inside parison 2, thereby greatlyincreasing the total rate of flow of fluid into parison 2. Parison 2 isblown to conform to the inner surface of mold 1 and then cools.

At the completion of the blowing cycle, the fluid through fluid supplyopenings 15a and 15b to small diameter hollow needle 6 and largediameter hollow needle 7 is stopped and fluid from fluid supply openings14a and 14b is started. This causes small diameter hollow needle 6 andlarge diameter hollow needle 7 to retract for the beginning of the nextcycle.

Referring to FIG. 11, an alternative embodiment includes a thinningcavity 5d in flash cavity 5 which reduces the thickness of parison 2 atthe site of large diameter hollow needle 7. Thinning cavity 5d isentirely within flash section 5c. Since the surface area of thinningcavity 5d is greater than the corresponding surface area of the hollowmold, the portion of parison 2 that is blown into thinning cavity 5d isthinner than the remainder of parison 2. Reducing the thickness ofparison 2 at the site of large diameter hollow needle 7 further reducesthe elasticity of the balloon wall and makes it easier for largediameter hollow needle to penetrate. A protrusion in the flash section(not shown) surrounding large diameter hollow needle 7 would have thesame effect as thinning cavity 5d of increasing the surface area.

In an application using an embodiment of the present invention, a moltenparison 2 extruded from an extruder is given a first blow through asmall diameter hollow needle 6 for about 3 seconds within a blow mold(split mold) using room temperature air at a blow pressure of 6 kg/cm².About one second after the first blow is begun, a second blow isperformed wherein a large diameter hollow needle 7 blows roomtemperature air at a blow pressure of 8 kg/cm² for about 9 seconds.After this, the pressurized air within the article is exhausted inapproximately 0.1 seconds.

Compared to the prior art blow molding method using only one hollowneedle, the blow molding method with the large and small hollow needlesof the present invention reduces blow time by about 11 seconds andexhaust time by about 1.9 seconds. The molding cycle is improved about60 percent.

Various conventional fluids can be used, and different fluids can beused for each hollow needle. Examples of conventional fluids includeair, either room temperature or refrigerated, and liquid or gaseouscarbon dioxide.

Having described preferred embodiments of the invention with referenceto the accompanying drawings, it is to be understood that the inventionis not limited to those precise embodiments, and that various changesand modifications may be effected therein by one skilled in the artwithout departing from the scope or spirit of the invention as definedin the appended claims.

What is claimed is:
 1. A method for blow molding hollow articles,comprising:interposing a thermoplastic polymer parison between first andsecond split molds; closing said first and second split molds therebysealing first and second ends of said parison; inserting a first hollowneedle into said parison in a flash section near said first end; blowinga fluid through said first hollow needle into said parison therebyforming a balloon by expanding said parison into a hollow space insidesaid closed split molds; inserting a second hollow needle into saidballoon in said flash section and between said first hollow needle andsecond end, said second hollow needle having a tip, a sidewall extendingfrom said tip and an orifice located in said sidewall; blowing saidfluid through said second hollow needle, out said orifice and into saidballoon thereby forming said hollow article, said hollow article havinga flash section and said orifice being positioned so that said fluid isdirected toward said second end and blowing said fluid so that saidfluid circulates inside said balloon; forming an exhaust opening in saidflash section of said hollow article; cooling said hollow article;exhausting said fluid from said hollow article through said exhaustopening; retracting said first hollow needle from said balloon or saidhollow article; retracting said second hollow needle from said hollowarticle; opening said first and second split molds; and removing saidhollow article from said first and second split molds.
 2. A methodaccording to claim 1, wherein an orifice diameter of said second hollowneedle is between 3 and 15 times an orifice diameter of said firsthollow needle.
 3. A method according to claim 1, wherein:said fluid isblown through said first hollow needle into said parison for a time L;said fluid is blown through said second hollow needle into said balloonfor a time Q; said time Q begins a time S after said time L begins; andsaid time L and said time Q overlap for a time W whereby L=S+W.
 4. Amethod according to claim 3, wherein said time S is between 0.2 secondsand 2 seconds.
 5. A method according to claim 3, wherein said time L isbetween 0.4 seconds and 12 seconds.
 6. A method according to claim 3,wherein said time Q is between 5 seconds and 30 seconds.
 7. A methodaccording to claim 3, wherein said time W is between 0.2 seconds and 10seconds.
 8. A method according to claim 3, wherein said blowing saidfluid through said second hollow needle into said balloon is axiallyoriented with a major axis of said hollow article.
 9. A method accordingto claim 3, wherein said first hollow needle and said second hollowneedle are substantially in parallel.
 10. A method according to claim 3,wherein said first hollow needle and said second hollow needle areangled to each other, whereby said first and second hollow needles areinserted into said balloon close to a same longitudinal position on anaxis of said balloon.